Synthesis and reactivity of group 6 and group 8 organometallic complexes with sulfur-containing ligands

Ph.DDOCTOR OF PHILOSOPH

Chloromethyl Acryl Reagents for Simple and Site-Selective Cysteine and Disulfide Modification

The generation of protein biotherapeutics with improved features compared to the synthetic drugs has received emerging interest. The conjugation of various synthetic functionalities to proteins provides access to new classes of protein conjugates, where the advantages from both the synthetic world and Nature can be combined in a synergistic fashion. Here, we reported that 2-chloromethyl acryl scaffold can serve as a simple yet versatile platform for synthesizing acrylamide or acrylate derivatives by coupling with different end-group functionalities (amino group or hydroxyl group) via a one-pot reaction. The chemical properties of the amide or ester linkage influence their inherent reactivity as bioconjugation reagents, which in turn allows synthetic customization of their features to achieve selective protein modification at cysteine or disulfide sites on demand. 2-Chloromethyl acrylamide reagents with amide linkage favors selective modification at cysteine site with high efficiency and the resultant bioconjugates exhibit superior stability compared to commonly employed maleimide-thiol conjugates. In contrast, 2-chloromethyl acrylate reagents bearing ester linkage can undergo two successive Michael reaction, allowing the selective modification of disulfides with high labelling efficiency and conjugate stability. These reagents could outperform widely applied maleimide reagents in terms of stability of the resultant bioconjugates without compromising on the ease of reagent preparation, reactivity and reaction speed. <br /

Mixed-Sandwich Cp*Cr Complexes Containing Poly(methimazolyl)borates (Cp* = C₅Me₅): Syntheses and Structural and Electrochemical Studies

Reaction of the scorpionate salts K­[HB­(mt)₃], Na­[H₂B­(mt)₂], and Li­[HB­(mt)₂(pz)] with [Cp*CrBr₂]₂ (<b>1</b>) gave the 15-electron Cr­(III) complexes [Cp*Cr­{κ³<i>-S</i>,<i>S′</i>,<i>S″</i>-HB­(mt)₃}]Br (<b>2</b>), [Cp*Cr­{κ²<i>-S</i>,<i>S′</i>-H₂B­(mt)₂}­Br] (<b>3</b>), and [Cp*Cr­{κ²<i>-S,S′</i>-HB­(mt)₂(pz)}­Br] (<b>4</b>) in moderate to high yields. These are the first examples of mixed-sandwich chromium­(III) complexes containing poly­(methimazolylborate) ligands. The tridentate coordination of the monoanionic ligands [H₂B­(mt)₂] in <b>3</b> and [HB­(mt)₂(pz)] in <b>4</b> can be effected by using a silver salt to remove the Br coligand, thus yielding the complexes [Cp*Cr­{κ³<i>-H</i>,<i>S</i>,<i>S′</i>-H₂B­(mt)₂}]­PF₆ (<b>6</b>) and [Cp*Cr­{κ³<i>-N</i>,<i>S</i>,<i>S′</i>-HB­(mt)₂(pz)}]­PF₆ (<b>7</b>). It was also found that, in the presence of acetonitrile, the reaction of <b>3</b> with AgPF₆ afforded [Cp*Cr­{κ²<i>-S</i>,<i>S′</i>-H₂B­(mt)₂}­(NCMe)]­PF₆ (<b>5</b>). The coordination geometries of all the complexes have been determined by X-ray diffraction analyses. Cyclic voltammetric studies of complexes <b>2</b>,<b> 4</b>, and <b>7</b> showed that the oxidation and reduction processes are chemically reversible and that the reduced and oxidized states of complexes <b>3</b>, <b>5</b>, and <b>6</b> are very short-lived

Mixed-Sandwich (Cp*/(HMB))Ru Complexes Containing Bis(methimazolyl)(pyrazolyl)borate (Cp* = η⁵-C₅Me₅, HMB = η⁶-C₆Me₆)

Reaction of the scorpionate salt Li­[HB­(mt)₂(pz)] (mt = <i>N</i>-methyl-2-mercaptoimidazol-1-yl, pz = pyrazolyl) with the organometallic complexes [Cp*RuOMe]₂ (Cp* = η⁵-C₅Me₅) (<b>1</b>) and [(HMB)­RuCl₂]₂ (HMB = η⁶-C₆Me₆) (<b>2</b>) gave the 18-electron Ru­(II) complexes [Cp*Ru­(κ³<i>-H,S,S′</i>)<i>-</i>{HB­(mt)₂(pz)}] (<b>3</b>) and [(HMB)­Ru­(κ³<i>-H,S,S′</i>)<i>-</i>{HB­(mt)₂(pz)}]­(<b>4B</b>)­PF₆ in moderate yields. In the absence of the PF₆^– anion, [(HMB)­Ru­(κ²-<i>S</i>,<i>S</i>′-{HB­(mt)₂(pz)})­(Cl)] [<b>4C</b>] was isolated as a coproduct with (<b>4B</b>)­Cl. These complexes are the first examples of organoruthenium­(II) complexes containing bis­(methimazolyl)­(pyrazolyl)­borate ligands. Isomers of <b>4B</b> were observed in solution, and the isomerization process was studied using variable-temperature ¹H NMR spectroscopy. The reactivity of <b>3</b> toward O₂ and CO was investigated, and in the process we isolated the first Ru­(IV) peroxo complex containing a poly­(methimazolyl)­borate ligand, [Cp*Ru­(κ²-<i>S</i>,<i>S</i>′-{HB­(mt)₂(pz)})­(η²-O₂)] (<b>5</b>), and a CO adduct, [Cp*Ru­(κ²-<i>S</i>,<i>S</i>′-{HB­(mt)₂(pz)})­(CO)] (<b>6</b>), respectively. The oxidation process was reversible, but treatment of <b>5</b> with CO converted it irreversibly to <b>6</b>. All the new compounds were fully characterized, including by X-ray diffraction analyses. Cyclic voltammetric studies were also conducted for complexes <b>3</b>, <b>5</b>, and <b>6</b>

Mixed-Sandwich (Cp*/(HMB))Ru Complexes Containing Bis(methimazolyl)(pyrazolyl)borate (Cp* = η⁵-C₅Me₅, HMB = η⁶-C₆Me₆)

Reaction of the scorpionate salt Li­[HB­(mt)₂(pz)] (mt = <i>N</i>-methyl-2-mercaptoimidazol-1-yl, pz = pyrazolyl) with the organometallic complexes [Cp*RuOMe]₂ (Cp* = η⁵-C₅Me₅) (<b>1</b>) and [(HMB)­RuCl₂]₂ (HMB = η⁶-C₆Me₆) (<b>2</b>) gave the 18-electron Ru­(II) complexes [Cp*Ru­(κ³<i>-H,S,S′</i>)<i>-</i>{HB­(mt)₂(pz)}] (<b>3</b>) and [(HMB)­Ru­(κ³<i>-H,S,S′</i>)<i>-</i>{HB­(mt)₂(pz)}]­(<b>4B</b>)­PF₆ in moderate yields. In the absence of the PF₆^– anion, [(HMB)­Ru­(κ²-<i>S</i>,<i>S</i>′-{HB­(mt)₂(pz)})­(Cl)] [<b>4C</b>] was isolated as a coproduct with (<b>4B</b>)­Cl. These complexes are the first examples of organoruthenium­(II) complexes containing bis­(methimazolyl)­(pyrazolyl)­borate ligands. Isomers of <b>4B</b> were observed in solution, and the isomerization process was studied using variable-temperature ¹H NMR spectroscopy. The reactivity of <b>3</b> toward O₂ and CO was investigated, and in the process we isolated the first Ru­(IV) peroxo complex containing a poly­(methimazolyl)­borate ligand, [Cp*Ru­(κ²-<i>S</i>,<i>S</i>′-{HB­(mt)₂(pz)})­(η²-O₂)] (<b>5</b>), and a CO adduct, [Cp*Ru­(κ²-<i>S</i>,<i>S</i>′-{HB­(mt)₂(pz)})­(CO)] (<b>6</b>), respectively. The oxidation process was reversible, but treatment of <b>5</b> with CO converted it irreversibly to <b>6</b>. All the new compounds were fully characterized, including by X-ray diffraction analyses. Cyclic voltammetric studies were also conducted for complexes <b>3</b>, <b>5</b>, and <b>6</b>

Mixed-Sandwich Cp*Cr Complexes Containing Poly(methimazolyl)borates (Cp* = C₅Me₅): Syntheses and Structural and Electrochemical Studies

Reaction of the scorpionate salts K­[HB­(mt)₃], Na­[H₂B­(mt)₂], and Li­[HB­(mt)₂(pz)] with [Cp*CrBr₂]₂ (<b>1</b>) gave the 15-electron Cr­(III) complexes [Cp*Cr­{κ³<i>-S</i>,<i>S′</i>,<i>S″</i>-HB­(mt)₃}]Br (<b>2</b>), [Cp*Cr­{κ²<i>-S</i>,<i>S′</i>-H₂B­(mt)₂}­Br] (<b>3</b>), and [Cp*Cr­{κ²<i>-S,S′</i>-HB­(mt)₂(pz)}­Br] (<b>4</b>) in moderate to high yields. These are the first examples of mixed-sandwich chromium­(III) complexes containing poly­(methimazolylborate) ligands. The tridentate coordination of the monoanionic ligands [H₂B­(mt)₂] in <b>3</b> and [HB­(mt)₂(pz)] in <b>4</b> can be effected by using a silver salt to remove the Br coligand, thus yielding the complexes [Cp*Cr­{κ³<i>-H</i>,<i>S</i>,<i>S′</i>-H₂B­(mt)₂}]­PF₆ (<b>6</b>) and [Cp*Cr­{κ³<i>-N</i>,<i>S</i>,<i>S′</i>-HB­(mt)₂(pz)}]­PF₆ (<b>7</b>). It was also found that, in the presence of acetonitrile, the reaction of <b>3</b> with AgPF₆ afforded [Cp*Cr­{κ²<i>-S</i>,<i>S′</i>-H₂B­(mt)₂}­(NCMe)]­PF₆ (<b>5</b>). The coordination geometries of all the complexes have been determined by X-ray diffraction analyses. Cyclic voltammetric studies of complexes <b>2</b>,<b> 4</b>, and <b>7</b> showed that the oxidation and reduction processes are chemically reversible and that the reduced and oxidized states of complexes <b>3</b>, <b>5</b>, and <b>6</b> are very short-lived

“Tag and Modify” Protein Conjugation with Dynamic Covalent Chemistry

The development of small protein tags that exhibit bioorthogonality, bond stability, and reversibility, as well as biocompatibility, holds great promise for applications in cellular environments enabling controlled drug delivery or for the construction of dynamic protein complexes in biological environments. Herein, we report the first application of dynamic covalent chemistry both for purification and for reversible assembly of protein conjugates using interactions of boronic acid with diols and salicylhydroxamates. Incorporation of the boronic acid (BA) tag was performed in a site-selective fashion by applying disulfide rebridging strategy. As an example, a model protein enzyme (lysozyme) was modified with the BA tag and purified using carbohydrate-based column chromatography. Subsequent dynamic covalent “click-like” bioconjugation with a salicylhydroxamate modified fluorescent dye (BODIPY FL) was accomplished while retaining its original enzymatic activity

Assembly of pH-Responsive Antibody-Inspired Protein-Drug Conjugates

With the advent of chemical strategies that allow the design of smart bioconjugates, peptide- and protein-drug conjugates are emerging as highly efficient therapeutics to overcome limitations of conventional treatment, as exemplified by antibody-drug conjugates. While targeting peptides serve similar roles as antibodies to recognize overexpressed receptors on diseased cell surfaces, peptide-drug conjugates suffer from poor stability and bioavailability due to their low molecular weights. Through a combination of a supramolecular protein-based assembly platform and a pH-responsive dynamic covalent linker, we devise herein the convenient assembly of a trivalent protein-drug conjugate. The conjugate mimics key features of antibody-drug conjugates such as (1) a multipartite structure, (2) peptide recognition sites arranged at distinct locations and at defined distances, (3) a high molecular weight protein scaffold, and (4) an attached drug molecule. These antibody-inspired protein-drug conjugates target cancer cells that overexpress somatostatin receptors, enable controlled release in the microenvironment of cancer cells through an entirely new dynamic covalent biotin linker and exhibit stability in biological media

Dual-Locked Macrocyclic “Turn-On” Drug for Selective and Traceless Release in Cancer Cells

Drug safety and efficacy due to premature drug release in the bloodstream and poor biodistribution remain challenging issues despite seminal advances in the field. To circumvent these limitations, we report a directed-macrocylization as a dual lock for camptothecin (CPT), a small molecule anticancer drug. In this way, the activity is “locked” within the cyclic structure by the redox responsive disulfide and pH-responsive boronic acid-salicylhydroxamate and turned on only in the presence of acidic pH and glutathione through traceless release. Notably the dual-responsive CPT is more active (100-fold) compared to the non-cleavable (closed) analogue. We further include bioorthogonal handle in the cyclic backbone for subsequent functionalization to generate cell-targeting peptide-macrocyclic and protein-macrocyclic CPTs for targeted, traceless drug release in triple negative metastatic breast cancer cells to inhibit cell growth in the low nanomolar concentration